1. Field of the Invention
The invention relates to a nanocomposite thermoelectric conversion material in which nano-sized phonon-scattering particles are dispersed in a matrix made of a thermoelectric conversion material, and a method of producing the same.
2. Description of the Related Art
A thermoelectric conversion material is an energy material that directly converts thermal energy to electric energy, based on two basic thermoelectric effects, that is, the Seebeck effect and the Peltier effect.
A thermoelectric generation device, which uses the thermoelectric conversion material, has many advantages as compared to conventional power generation technologies. For example, the thermoelectric generation device has a simple structure, and is robust and highly durable. The thermoelectric generation device does not have a movable member. The micro-sized thermoelectric generation device is easily produced. The thermoelectric generation device does not require maintenance. The thermoelectric generation device is highly reliable, has a long lifespan, does not cause noise, and does not cause contamination. The thermoelectric generation device uses low-temperature waste heat.
A thermoelectric cooling device, which uses the thermoelectric conversion material, also has advantages as compared to conventional compression cooling technologies. For example, the thermoelectric cooling device does not require chlorofluorocarbon, and does not cause contamination. The small-sized thermoelectric cooling device is easily produced. The thermoelectric cooling device does not have a movable member, and does not cause noise.
Therefore, particularly because energy-related issues and environment-related issues have recently become more serious, it is expected that the thermoelectric conversion material will be put to practical use in fields of aerospace, national defense, construction, geological observation, weather observation, medical care, hygiene, microelectronics, and the like. Also, it is expected that the thermoelectric conversion material will be used for various purposes, for example, for the purpose of using waste heat in petrochemical industry, metallurgy, and electric power industry.
A power factor P=S2σ, and a nondimensional performance index ZT=(S2σ/κ) T are used as indices for evaluating the performance of the thermoelectric conversion material. In this case, S represents a Seebeck coefficient, σ represents an electric conductivity, κ represents a thermal conductivity, and T represents an absolute temperature. That is, in order to obtain a good thermoelectric characteristic, the Seebeck coefficient S and the electric conductivity σ need to be high, and the thermal conductivity κ needs to be low.
To scatter phonons, which conduct heat, is effective for decreasing the thermal conductivity κ. Thus, a composite thermoelectric conversion material, in which particles used for scattering the phonons (hereinafter, referred to as “phonon-scattering particles”) are dispersed in a matrix made of a thermoelectric conversion material, has been proposed.
Japanese Patent Application Publication No. 2000-261047 (JP-A-2000-261047) describes a composite thermoelectric conversion material in which ceramic particles, which serve as phonon-scattering particles, are dispersed in a matrix made of a thermoelectric conversion material CoSbx (2.7<x<3.4). The size of the ceramic particles ranges from sub-micron size to several hundred micron size. The publication No. 2000-261047 also describes a production method in which raw material powder of the matrix is mixed with ceramic powder, the mixed powder is shaped, and calcination is performed.
In the publication No. 2000-261047, consideration is not given to an interface between the matrix and the phonon-scattering particles (ceramic particles). In an example, a thermal conductivity is 1.8 to 3 W/km, and is lower than approximately 5 W/km of a thermoelectric conversion material CoSbx that does not include dispersed phonon-scattering particles. However, the thermal conductivity is required to be further decreased.
In Japanese Patent Application Publication No. 2009-147145 (JP-A-2009-147145), it is described that when there is roughness at an interface between a matrix and phonon-scattering particles (i.e., when interface density is increased), the phonon-scattering effect caused by the phonon-scattering particles is increased. In the publication No. 2009-147145, Al2O3 nanoparticles or SiO2 nanoparticles, which serve as phonon-scattering particles, are dispersed in a thermoelectric conversion material CoSb3-based matrix, and interface roughness between the matrix and the nanoparticles is used. In this case, it is not possible to further increase the interface roughness.